Approximately 48 million people in the US are served by private wells, many of which are untreated. We estimate that 1.29 million cases of acute gastrointestinal illness (AGI) per year are attributed to consuming water from untreated private wells in the US. These cases of AGI can cause a significant burden in terms of health care costs and lost work/ school days. This impact is magnified when accounting for vulnerable populations such as children under the age of 5, the elderly and the immuno-compromised We propose to perform the first randomized controlled trial (RCT) to investigate whether consuming water treated by ultraviolet light (UV) compared to consuming untreated private well water decreases the incidence of AGI in children under 5. We will investigate the presence of enteric viral and bacterial pathogens in water and stool samples of a sub-sample of children exposed and unexposed to untreated groundwater. There are no prior RCTs or established causal links between AGI and the consumption of untreated water from private wells despite the fact that waterborne pathogens have been recovered in groundwater supplies, including in deep aquifers. The proposed grant submission is for a R34 planning grant for the above RCT. The specific aims of the planning grant (A1) and the trial (A2 & A3) are as follows: A1- With the guidance of an interdisciplinary advisory committee of experts, we will use the planning grant to: a) identify enrollment sites and community collaborators, b) select appropriate UV treatment devices, c) assess feasibility of novel data collection tools, d) finalize the trial protocol and e) prepare the manual of procedures and a detailed budget to execute Aims 2 & 3. A2- Quantify the proportional attributable risk of endemic AGI associated with consumption of untreated private well water compared to water treated by UV in a population of children under the age of 5 in Pennsylvania. A3a- Identify, quantify and compare the presence of viral pathogens in treated and untreated private wells A3b- Examine the presence of the same viral pathogens in child stool samples comparing children drinking untreated compared to treated private wells. A3c- Explore the presence of bacterial pathogens in treated and untreated private well water and corresponding child stool samples of sick children. The trial results will help inform AGI burden estimates and policy decisions regarding management of private water supplies across the US.